A multiple-input-multiple-output (MIMO) communication system employs multiple transmit antennas in a transmitter and multiple receive antennas in a receiver for data transmission. A MIMO channel formed by the transmit and receive antennas may be decomposed into independent channels, wherein each channel is a spatial sub-channel (or a transmission channel) of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
MIMO techniques are adopted in wireless standards, such as IEEE802.11n, for high data rate services. In a wireless MIMO system, multiple antennas are used in both transmitter and receiver, wherein each transmit antenna can transmit a different data stream into the wireless channels whereby the overall transmission rate is increased.
There are two types of MIMO systems, known as open-loop and closed-loop. In an open-loop MIMO system, the MIMO transmitter has no prior knowledge of the channel condition (i.e., channel state information). As such, space-time coding techniques are usually implemented in the transmitter to combat fading channels. In a closed-loop system, the channel state information (CSI) can be fed back to the transmitter from the receiver, wherein some pre-processing can be performed at the transmitter in order to separate the transmitted data streams at the receiver side.
Such techniques are referred to as beamforming techniques, which provide better performance in desired receiver's directions and suppress the transmit power in other directions. In fact, beamforming techniques are considered as promising candidates for IEEE 802.11n (high throughput WLAN) standard. In such a system, impairments such as channel estimation errors and frequency offset errors will degrade the system performance significantly. As such, there is a need for a beamforming transceiver architecture which performs processing on the preamble at the transmitter to reduce performance degradation due to the above impairments.